For many years, it has been conventional practice in providing for the measurement and adjustment of the ignition angle of an internal combustion engine to provide a mark on the pulley of the engine which can be observed. A stroboscopic lamp is directed on the pulley to illuminate the timing mark. This stroboscopic lamp is energized under the control of ignition pulses from the ignition system. By adjusting the delay interposed between an ignition pulse from the ignition system and the firing of the lamp until the timing mark appears to be at the top dead center position, it is possible to determine the amount of ignition timing.
In recent years, it has become increasingly common to provide the engine with some means, often a notch in the pulley, for producing an electrical pulse when the crankshaft is in a predetermined position. By comparing the position of this pulse with the position of a pulse derived from the voltage applied to a particular plug, such as the "No. 1" plug of the engine, it is possible to determine the amount of angular displacement of the ignition. Unfortunately, the different manufactures of automotive vehicles place the notches or other means indicative of the position of the pulley at different points with respect to top dead center. This is partly due to the physical limitations in the placement of the pulse generator cooperating with the notch or other position indicator, due to the auxiliary equipment being employed. It is also due, in part, to the fact that different engineers have different opinions as to the most desirable location for such a pulse generator, usually a magnetic pulse generator. As a result, if a timing measurement device is to be used with different types of automobile engines, some means must be provided for correcting for these differences in displacement of the magnetic pulse generator from top dead center, this displacement commonly being referred to as the reference angle.
It has been previously proposed to generate a series of pulses each being initiated with the occurrence of an ignition pulse supplied to the No. 1 plug and each terminating upon a signal received from the magnetic pulse generator, algebraically adding to this series of pulses a voltage indicative of the reference angle and then integrating the resultant pulses to determine the reference angle. While this arrangement is quite satisfactory, it basically provides an analog output and does require calibration arrangements if the indication of timing angle is to be correct.
It has also been proposed in the Crawford et al U.S. Pat. No. 3,753,082 and the Abnett et al U.S. Pat. No. 3,768,004 to provide a pulse generator which is operated at a frequency related to the speed of engine revolution so that there is a constant number of pulses per engine revolution. By counting the number of pulses occurring between the firing of a particular plug and the signal from the reference generator, it is possible in this manner to obtain a count equal to the reference angle. The apparatus described in the above mentioned patents, however, is particularly designed for use in connection with production lines in which the reference angle is always the same and in which it is known with a fair degree of certainty whether the ignition is advanced or retarded. A switch is thus provided for selecting whether the apparatus is measuring the timing angle where the ignition is advanced or the timing angle where the ignition is retarded. When the ignition is advanced, the counting starts with the firing of the reference plug; when the ignition is retarded, the counting starts with a pulse related to the signal from the reference generator. This presupposes a knowledge of which condition exists. While this is possible in a production line, it is not possible where equipment is to be used with a wide variety of automobiles, as is the case in a typical automotive diagnostic shop or repair shop. Furthermore, the apparatus depends upon the use of a signal from the high voltage ignition source connected to the distributor mechanism since the apparatus requires a signal corresponding to the firing of each cylinder. This is done so that it is possible to measure the timing advance of each plug with respect to a pulse simulating the angular position from top dead center. The difficulty with such an arrangement is that with certain modern ignition systems, it is very difficult to connect to the ignition system at a point at which such a high voltage signal is available. While the apparatus of the above mentioned patents may be suitable for the testing of engines on a production line in which the reference angle is relatively fixed and in which it is known with a fair degree of certainty whether the ignition is advanced or retarded, the apparatus for these reasons and for a variety of other reasons is not suitable for use in testing engines of a great variety of automobiles.